I like this a lot. Reminds me of their relationship in the short story The Diary of Luke Castellan, in the Demigod Diaries. I used to really dislike Thuke, but since I read that, I like it much more... there was a lot of chemistry between them
It's not a fanfic, it's officially by Rick Riordan. It's an excerpt of Luke's diary shortly before he and Thalia meet Annabeth. The Demigod Diaries is another short companion book, much like The Demigod Files.